South Korea's foreign minister has urged North Korea not to conduct a nuclear test, warning that to do so would "shake the foundation of the global non-proliferation system."

Foreign Minister Ban Ki-moon warned Wednesday that a North Korean nuclear test would have much more serious consequences than Pyongyang's missile tests in July.

"If North Korea conducts a nuclear test, it would create a threatening situation that would shake the foundation of the global non-proliferation system and further isolate North Korea," said Ban.

U.S. news reports last week quoted U.S. officials as saying suspicious activities had been observed at a possible North Korean underground test site.

Ban says Seoul is closely monitoring the situation and sharing information with other nations. A nuclear test by Pyongyang would be the latest incident in a long-running international dispute over North Korea's weapons programs.

North Korea claims it has nuclear weapons, but there has been no independent verification of this assertion, and the North has never tested a nuclear device.

What is known is that Pyongyang has extracted enough plutonium from a nuclear reactor to make several nuclear weapons, and might be attempting to enrich uranium to make nuclear fuel as well.

Since 2003, the U.S., South Korea, Japan, China and Russia have been urging the North to abandon such weapons programs, in exchange for economic aid and security guarantees.

But after attending several rounds of six-party negotiations, Pyongyang has boycotted further talks, saying it will not return unless the United States lifts sanctions aimed at curbing alleged counterfeiting and money laundering by the North.

In an act of defiance, Pyongyang test-fired seven short- and long-range missiles last month. That led to sanctions by the United Nations, and a halt on South Korea's much-needed economic aid to the North.

Still, Pyongyang has shown no sign of giving in to the international pressure.

South Korea and the United States have urged the North to resume negotiations without conditions, saying the sanctions have nothing to do with the nuclear talks.

President Bush has said that the possibility of a nuclear test is a reminder that North Korea poses a threat. He said the United States expects the other countries in the six-party talks "to help rid the world of the threat."